Methods and apparatus for making ring-shaped parts out of a composite material, as well as preforms therefor

ABSTRACT

Preforms for producing annular parts of composite material are produced byinding a strip (1) of fibrous material around an elliptical mandrel (10) to produce an elliptical sleeve which can be cut obliquely, cutting taking place either before or after one or more densification steps. The cutting plane is inclined relative to the right cross section of the elliptical sleeve, and the inner and outer portions of the cut rings are machined to obtain circular elements. Such elements can produce annular parts of composite material having a reduced tendency to delanination under circumferential stress. The superposed layers are needling together during winding. A uniform needling density per unit area can be obtained by controlled displacement of the needle board (5) and of the elliptical mandrel (10) relative to each other as the mandrel turns, to compensate for the eccentricity of the cross section of the mandrel. These methods are particularly suitable for producing brake disk preforms.

BACKGROUND OF THE INVENTIONS

The invention relates to methods and to apparatuses for producingannular parts of composite material and preforms for such parts, andalso to the parts and to the preforms themselves. More particularly, theinvention relates to methods and apparatus for producing preforms bywinding a fibrous strip on a mandrel, the wound layer assembly beingintended to be cut into rings either before or after densification by amatrix.

Conventional methods of producing preforms for composite material partsconsist in stacking flat layers of fibrous material, cutting ormachining the assembly to obtain a preform with the desired shape, andthen densifying the preform. When the part to be produced is a brakedisk or some other annular part, about half of the weight of thematerial is lost when producing annular preforms from an assembly ofstacked layers.

A number of proposals have been put forward to reduce such waste. Oneproposal consists in assembling an annular preform to be densified fromlayers of fibrous material each in the form of juxtaposed sectors, thelayers then being stacked. Such a method reduces waste but does notavoid it.

A further proposal made in French patent application FR-A-2 506 672 isdescribed below with reference to FIGS. 1A to 1D. Annular or cylindricalelements are produced by winding a fibrous strip on a cylindricalmandrel (FIG. 1A) to produce a cylindrical sleeve (FIG. 1B). Duringwinding, the superposed layers are connected together by needling. Thecylindrical sleeve can be cut perpendicularly to its axis to obtainannular preforms to be densified (FIG. 1C).

A method similar to the above has also been described in French patentapplication FR-A-2 584 107.

That method avoids wasting material, but the parts made from preformsproduced by that method have disadvantageous features which come tolight during service. Brake disks are subjected to shear stresses in atangential direction during use. The stresses are particularly high inthe notches formed in the inner or outer border (FIG. 1D) to connect thedisk with a moving or a fixed portion. Such shear stresses E can causethe part to delaminate, i.e., it is destroyed by the layers in thepreform separating.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a method and apparatus forproducing preforms for annular parts of composite material which have ahigh resistance to delamination, and which limit the loss of materialcaused by carrying out the method.

In the method and apparatus of the invention, a strip of fibrousmaterial is wound around an elliptical mandrel to form an ellipticalsleeve. The elliptical sleeve can be cut obliquely to produce annularpreforms for densification or it can itself act as the preform fordensification, cutting being delayed until the end of one or more of thedensification steps. The cutting plane is inclined to the right crosssection of the elliptical sleeve, i.e. the cutting plane is notperpendicular to the sleeve axis. The inner and outer portions of thecut rings are machined to obtain circular elements.

The method and apparatus described above exploit the fact that theprojection of an ellipse onto a plane passing through its major axis andat an angle α to the plane of the ellipse is a circle of diameter equalto the major axis of the ellipse. The angle a can be calculated usingthe following formula:

    cos α=b/a

where:

2a=the major axis of the ellipse and 2b =the minor axis of the ellipse.

The principle of this aspect of the invention is illustrated in FIGS. 2Ato 2D.

FIG. 2A shows a strip of fibrous material being wound around anelliptical mandrel and FIG. 2B shows the cutting plane of the ellipticalsleeve produced by the sleeve produced by the assembly of superposedlayers. In this example, the ring cut from the sleeve is perfectlycircular at its half-width, but elliptical at the inner and outerperipheries. The cutting plane can also be selected so that the ring iscircular at its inner or outer periphery or at another distance from theperipheries of the ring. (In theory, a cutting plane could be selectedwhich would render the cut ring elliptical throughout its width but thiswould increase the amount of material which would have to be removed toobtain a circular element).

Following cutting, the inner and outer peripheries of the ring aremachined to produce an element which is circular overall (see FIG. 2C).During use of a part produced from such an element, a shear stress E ina tangential direction is no longer in a direction which encouragesdelamination of the layers making up the preform of the part (see FIG.2D).

In this aspect of the invention, preforms for producing annular partscan be produced by a method which results in minor losses of materialand these preforms can result in annular parts having a reduced tendencyto delaminate during use.

The closer the cutting angle relative to the right cross section of thesleeve is to 45°, the more resistant are the parts cut from this sleeveto delamination under shear in a direction tangential thereto. However,material wastage increases with increase in this angle (hereinaftertermed the "cutting angle"). Thus the cutting angle must be optimized toobtain parts which perform well mechanically and which also haveacceptable material losses. The cutting angle is preferably in the range10° to 45°. When a cutting angle of 45° is used, material wastage isfairly high, about 35%. It is thus more advantageous to use a cuttingangle in the range 10° to 30°.

This first aspect of the invention also encompasses a method ofproducing annular parts of composite material, comprising theabove-described method of producing a preform plus steps of densifying,cutting, and machining the preform. Additional machining of the part maybe necessary in order to adapt the part to a specific use, for exampleas a brake disk.

Brake disk preforms are advantageously produced by needlingtwo-dimensional fabric formed from yams or tows of pre-oxidizedpolyacrylonitrile (PAN), carbon roving (application FR-A-2 669940) orhybrid yams (French patent application, number 95 06 200). In the priorart, two-dimensional fabrics, for example cloth or sheets of tows oryams, are needled to each other to form a slab from which annular brakedisk preforms are cut.

When materials are to be exposed to high thenmomechanical stresses, asis generally the case for brake disks, it is important that theproperties are kept constant throughout the mass in order to avoidnon-uniformity in mechanical and tribological properties. It is thusimportant that the needling density is kept uniform in a sleeve which isto act as a preform or a source of preforms for brake disks. However, ifa sleeve is produced by winding around an elliptical mandrel, needlingcan prove to be difficult.

In the known method described in FR-A-2 584 107 where a fibrous strip iswound around a cylindrical mandrel, each wound layer is needled by aneedle board as soon as it is deposited onto the mandrel. The mandrel isdriven in rotation about its axis, the position of that axis beingfixed, and the needle board which extends parallel to a generatrix ofthe mandrel is reciprocated in a direction perpendicular to the mandrelaxis. The amplitude of the displacement of the needle board is constant.In order to ensure a uniform needling depth in the wound layersthroughout the method, the average distance between the needle board andthe mandrel axis is increased as the cumulative thickness of the layersincreases.

Such a method cannot be used directly with an elliptical mandrel.

In a further aspect, the invention provides methods and apparatuses forproducing preforms for producing annular parts, in which a strip offibrous material is wound around an elliptical mandrel, and thesuperposed layers are needled in a uniform manner.

To this end, a method and apparatus are provided in which, according tothe invention, a strip of fibrous material is wound around an ellipticalmandrel and the wound layers are needled using a needle board while thedisplacement between the mandrel and the average position of the needleboard is controlled so as to compensate for the eccentricity of themandrel cross section. The displacement between the needle board and themandrel is preferably such that the needle board comes into contact withthe layers to be needled in a plane (the needling plane) which is fixedrelative to the average position of the needle board.

Control of the needle board is simplified if the needling frequencyremains constant, resulting in a need to keep the relative velocitybetween the needle board and the surface to be needled constant in orderto obtain a constant needling density per unit area. It is alsoessential that at the moment of contact, the needling surface of theneedle board is in a plane which is tangential to the facing needlingsurface in order to obtain a constant needling density per unit area.Displacement of the elements in order to satisfy these conditions isfacilitated if the rate of displacement of the needle board is keptconstant and the rotation speed of the mandrel and/or the rate ofdisplacement of its axis is varied.

In the following description, displacements of the needle board and/orthe axis of the mandrel are described. It should be understood that eachdisplacement of the needle board can be replaced by an appropriatedisplacement of the mandrel or by a combination of displacement of theneedle board and of the mandrel, and vice versa. The important point isto produce a relative motion between the needle board and the surface tobe needled which results in uniform needling of the wound layers.

However, as is well known in producing needled preforms, the needles inthe board must not strike at exactly the same place at each turn of themandrel since such a method would produce weakened zones in the sleevewhich was produced. The slight offset required to avoid this can beachieved by displacing the needle board (or the mandrel) in alongitudinal direction relative to the mandrel (i.e., in a directionparallel to the axis of rotation of the mandrel).

In a first aspect of the invention, in a first implementation, anelliptical mandrel is driven in rotation about its axis so as to wind astrip of fibrous material around its periphery and, at the same time,the axis of the mandrel periodically moves towards and away from aneedling plane along a path which is perpendicular to the needlingplane. The curved surface of the outer layer wound on the mandrel isflush with the needling plane, the region of contact being a line. Thedisplacement of the axis of the mandrel is regulated so as to keep thecurved surface in a position such that the needling plane is tangentialthereto at the contact line. This means that the portion of the curvedsurface which is flush with the needling plane slides in this plane withreciprocating motion. The needle board facing the curved surface of themandrel moves towards and away from the needling plane so as to strikeand pierce the outer layers wound around the mandrel. The needle boardis also displaced with reciprocating motion in the needling plane inorder to follow the motion of the contact line of the surface of thewound layers with the needling plane. At the moment of contact, theneedling surface of the needle board is oriented in a direction which istangential to the facing needling surface.

This first implementation can also be carried out by keeping the axis ofrotation of the mandrel fixed. The relative motion required between theneedle board and the mandrel is produced by displacement of the needleboard alone.

In a second implementation, the elliptical mandrel is still driven inrotation about its axis and, at the same time, the axis of the mandrelperiodically moves towards and away from a needling plane along a pathwhich is perpendicular to the needling plane. In this secondimplementation, the region where the curved surface of the outer woundlayer is flush with the needling plane is not displaced in this plane;but the orientation of the curved surface relative to the needling planechanges periodically as the mandrel turns. Thus the needle board doesnot need to be displaced in the needling plane. It is sufficient thatthe orientation of the needling surface of the needle board oscillatesperiodically so that it is in a plane which is tangential to the outersurface of the wound layers at the moment of contact between the needleboard and the layers to be needled.

In a third implementation, the elliptical mandrel is driven in rotationabout its axis and, at the same time, the axis of the mandrel isdisplaced to follow a path formed by two half-ellipses. Thisdisplacement is such that at any time, a portion of the curved surfaceof the outer wound layer is in a fixed location in the needling plane,which plane is tangential to that part of the curved surface. The needleboard strikes at this fixed location in the needling plane and theorientation of the needling surface can remain fixed.

The first of these three implementations is preferred because of itsrelative operational simplicity.

In a further aspect of the invention, preforms for producing annularparts can be produced by winding a strip of fibrous material on anelliptical mandrel, with simultaneous and uniform needling of the woundlayers, to produce an elliptical mandrel which can be cut on a slant.

This second aspect of the invention also provides a method of producingannular parts of composite material comprising the above-describedmethod of producing a preform plus steps of densifying, cutting, andmachining the preform.

The densification step in the method of manufacturing annular parts inaccordance with the first and second aspects of the invention can becarried out using any of the known methods. Densification can be begunbefore or after removing the sleeve from the mandrel. The sleeve canalso be cut into annular parts before densification or after one or moreof the densification steps. Preferably, at least the first densificationstep is carried out before the sleeve is cut because the material whichhas been consolidated in this way is stronger after the firstdensification step.

One known densification method is the conventional isothermal isobaricchemical vapor infiltration method. The element to be densified isplaced in a vessel into which a gas is introduced, which gas, underpredetermined temperature and pressure conditions, produces the materialconstituting the matrix by means of its constituents decomposing orreacting together. In the conventional method, the vessel includes asusceptor, generally of graphite, which delimits an infiltration chamberand which is coupled with an inductor which surrounds the vessel. Whenthis method is applied to producing annular parts from a sleeve, thesleeve is generally cut before densification, so that the gas caninfiltrate the internal pores of the parts more easily and uniformly.However, this gives rise to repeated interruptions of densification toenable the faces of the parts can be skinned to re-open the porescompletely and allow the gas to penetrate once again.

With elliptical sleeves, at least partial densification of the sleevecan be carried out by a temperature gradient type chemical vaporinfiltration method before the sleeve is cut. In this method, the sleevecan be heated by inductive coupling between the inductor and a centralcore on which the sleeve is positioned. Thus a temperature gradient isestablished between the inner surface of the sleeve in contact with therotor, which is the hottest surface, and the outer surface.Densification is then encouraged in those parts of the sleeve which arefurthest from the outer surfaces, and the risk of premature blocking ofthe superficial pores by unwanted deposits is reduced, eliminating theneed for skinning. It should be noted that the sleeve can also be heatedby direct inductive coupling between the inductor and the sleeve when itis of a suitable nature (for example, when the sleeve is of carbon). Atemperature gradient chemical vapor infiltration densification method isdescribed in French patent application FR-A-2 711 647, for example.

In order to further reduce the loss of material in producing annularparts in accordance with the first and second aspects of the invention,an elliptical cross section sleeve can be produced, both ends of whichare planar faces which are inclined relative to a right cross section ofthe sleeve, i.e., the faces are parallel to the envisaged cutting planefor the sleeve.

Further features and advantages of the present invention will becomeapparent from the following description which is made by way ofindication and is not limiting, and made with reference to theaccompanying drawings in which:

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 (FIGS. 1A to 1D) illustrates a known method of producing annularpreforms for annular parts of composite material, the method using acylindrical mandrel;

FIG. 2 (FIGS. 2A to 2D) illustrates a method of the present inventionfor producing annular parts of composite material, the method using anelliptical mandrel;

FIG. 3 is a flow chart showing typical steps in producing an annularpart using the methods of the invention;

FIG. 4 (FIGS. 4A to 4C) illustrates cutting an elliptical sleeve inaccordance with the invention to obtain an annular element;

FIG. 5 (FIGS. 5A to 5C) illustrates an example of cutting an ellipticalsleeve of the invention to obtain an annular element with pre-determineddimensions;

FIG. 6 indicates the dispositions of an elliptical mandrel and theneedle board in a first method of winding a strip on an ellipticalmandrel of the invention;

FIG. 7 is a diagram illustrating the disposition of the ellipticalmandrel in the first winding method;

FIG. 8 is a diagram illustrating a second method of winding a strip onan elliptical mandrel;

FIG. 9 indicates the positions of an elliptical mandrel and the needleboard in a third method of winding a strip on an elliptical mandrel ofthe invention;

FIG. 10 is a diagram illustrating the displacement of the ellipticalmandrel in the third winding method; and

FIG. 11 is a side view of an embodiment of apparatus for carrying outthe first implementation of the winding and needling method of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Producing a preform for producing a composite material annular partusing the methods of the present invention and producing the part itselfcomprise several steps, as shown in FIG. 3.

A first step E1 consists in winding a strip of fibrous material aroundan elliptical mandrel. The fibrous material strip may consist in anyknown material for producing composite material parts, or a mixture ofsuch materials. The fibers are in a suitable form (sheets, strips, wovencloth, etc., of tows, continuous yams, roving, etc.). The materialconstituting the fibers depends on the envisaged application. For brakedisks, carbon fibers are preferably used, but other materials may besuitable, in particular ceramics.

The wound layers are needled as they are deposited on the mandrel (stepE1'). As will be described below, several methods are possible forensuring that the needling density is uniform. When the assembly oflayers wound on the mandrel has reached a desired thickness, winding isstopped.

The elliptical sleeve thus produced is preferably then densified to fillthe internal pores thereof (step E2), for example by chemical vaporinfiltration, and cut on a slant to obtain rings (step E3). Theelliptical sleeve thus constitutes the preform to be densified.

The sleeve can also be cut before starting densification; in this casethe cut fibrous rings constitute the preforms to be densified. This ispossible only when the elliptical sleeve is sufficiently strong for itto be possible to withdraw the mandrel. Usually, needling the woundlayers is sufficient to produce a sleeve with the required strength. Thesleeve can also be produced from layers of resin-impregnated fibrousmaterial, the sleeve being treated before withdrawing the mandrel toform a single piece from the wound layers by polymerizing the resin.

If the elliptical sleeve is not strong enough to be removed from themandrel after winding, then densification can be started and continueduntil the sleeve has consolidated sufficiently (step E2'). At this time,the mandrel can be withdrawn and the sleeve can be cut into rings (stepE3'). The pre-densified elements thus produced could, after machining,be returned to the densification unit to finish densification (stepE2").

If the conventional isothermal chemical vapor infiltration methodalready described is used, the densified parts would be more uniform ifthe mandrel were to be withdrawn and the preforms cut beforedensification or at the latest during the densification process. Incontrast, the sleeve can be left uncut until densification is completeif the above-described temperature gradient method is used; preformdensification is uniform and material loss is reduced because noskinning is required.

The rings cut from the elliptical sleeve are not perfectly circular. Therings have to be machined in order to give them a perfectly annularshape (step E4).

One or more additional steps are necessary for producing an annularpart, such as a brake disk, from the annular element described above.The part must, for example, undergo final machining in order to make itsuitable for its end use (step ES). When the parts are for use as brakedisks, this final machining forms notches in the inner or outerperiphery of the parts.

One of more of the steps described above can be carried out using one ormore robots. The use of robotic means is particularly envisageable whenproducing the elliptical sleeve in the case of simultaneous winding andneedling. It is further noted that the mandrel can be provided with aprotective coating such as felt, into which the needles can penetratewithout being damaged when needling the first layers, as described inFR-A-2 584 107 cited above.

Before describing apparatus for carrying out the methods of theinvention, more details are now given concerning the steps of cuttingthe elliptical sleeve and the simultaneous winding and needling steps.

Cutting the sleeve is explained below with reference to FIGS. 4 and 5.

FIGS. 4A and 4B illustrate an elliptical sleeve 2 having a central hole3 (corresponding to the elliptical mandrel used for winding). FIG. 4A isan end view and FIG. 4B is a side view. In this example, the cuttingplane makes an angle of 30° relative to a right cross section of thesleeve. Lines AD and BC show two cutting planes delimiting a ring 4.

Cutting elliptical sleeve 2 makes use of the fact that the projection ofan ellipse in a plane passing through the major axis thereof and makingan angle α relative to the plane of the ellipse is a circle of diameterequal to the major axis of the ellipse. The relationship between theangle a and the dimensions of the ellipse is given by the formula:

    cos α=b/a

where 2a =the major axis of the ellipse; and

2b =the minor axis of the ellipse.

When the layers of material are wound onto an elliptical mandrel, theratio b/a of the ellipse defined by the outer layer differs from that ofthe ellipse defined by the mandrel (see FIG. 4A). Thus in a cuttingplane which is inclined relative to the plane normal to the axis of themandrel, a circular shape is obtained at the inner periphery of thesleeve but an elliptical shape is obtained at the outer periphery of thesleeve. The eccentricity of a ring cut from the elliptical sleeve isthus not constant throughout the width of the ring. The cutting angle αis preferably selected to produce a circular shape either at the innerperiphery of the cut ring (at the mandrel) or at the outer periphery ofthe cut ring (at the outside of the sleeve), or at a distanceintermediate between the inner and outer peripheries, and the outerand/or inner periphery of the ring is/are machined to obtain a circularshape.

Further, although the faces of each ring corresponding to the cuttingplanes are parallel, they are offset by a distance m. This means thatthe curved surfaces of the peripheries of the ring (inner and outer) arenot perpendicular to the faces corresponding to the cutting planes (seeFIG. 4C). This is an additional reason for machining the cut ring inorder to produce an exact annular shape.

FIG. 4C shows a ring 4 cut using a cutting plane which leads to acircular shape in this plane half way between the inner and outerperipheries of the ring, i.e., at the half-width of the ring. The dottedlines in the figure correspond to the portions of the ring which must beremoved to produce an element with a truly annular shape.

As explained above, with an annular preform intended to produce annularparts which can be subjected to shear stresses in a tangentialdirection, the cutting angle is selected to optimize the balance betweenmaterial wastage and the desired resistance to delamination in theannular parts. In this respect, a cutting angle in the range 10 ° to 45°is preferred, and an angle in the range 10° to 30° has been shown to behighly advantageous as regards reducing material loss. Once the cuttingangle has been selected, the eccentricity of the mandrel can be selectedso as to obtain a circular shape in the cutting plane at the desiredwidth.

When selecting the dimensions of the elliptical mandrel, and thethickness of the sleeve obtained by winding for producing preforms withdefined inner and outer dimensions, the effect of machining on the innerand outer dimensions of the cut rings must be taken into account. FIG. 5shows an example of cutting.

FIG. 5A shows the shape of cut ring 4 in any one of cutting planes AD orBC of FIG. 4 (the shape is the same along the thickness of the ring). Aportion at the half-width of the cut ring has a circular shape (seecircle c_(r) in FIG. 5). The outer and inner peripheries of the ring areelliptical so the ring peripheries must be machined to obtain a circularshape (around outer circle c_(e) and inner circle c_(i) in the figure).

FIGS. 5B and 5C show diagrams indicating the offset between the innercircles (c_(i) ^(AD), c_(i) ^(BC)) and outer circles (c_(e) ^(AD), c_(e)^(BC)) defined in planes AD and BC of FIG. 4B; this offset existsbetween the front and rear faces of ring 4. The relationships betweenthe dimensions of the elliptical sleeve and the cutting angle and thedimensions of the annular element obtained after machining ring 12(removal of the shaded portions in FIG. 5A) can be seen immediately inFIG. 5. If 2a₁ and 2b₁ are the lengths of the major and minor axis ofthe ellipse defined by the inner periphery, and 2a₂ and 2b₂ are thelengths of the major and minor axis defined by the outer periphery ofthe sleeve in the plane normal to the axis thereof, α is the cuttingangle relative to the plane normal to the sleeve axis, d and D are theinner and outer diameters of the annular preform to be produced and e isthe thickness thereof, we have:

    d≧2a.sub.1 +x

    ≧2a.sub.1 +e. tan α

and

    D≦2a.sub.2 -x

    ≦2a.sub.2 -e. tan α

If the desired dimensions of the annular preforms are d=250 mm, D=450 mmand e=25 mm, and if the cutting angle is 30°, then the ellipticalmandrel must have a major axis 2a₁ no greater than 225 mm and theellipse defined by the outer surface of the sleeve must have a majoraxis 2a₂ not less than 475 mm. Further, because the relationship cosα=b/a applies to the half-width of the sleeve, using the values 2a₁ =225mm and 2a₂ =475 mm:

    cos 45°=1/2(b.sub.1 +b.sub.2)/1/2(a.sub.1 +a.sub.2)

    0.707=1/2(b.sub.1 +b.sub.2)/175

    b.sub.1 +b.sub.2 ≈247.5

Since

2b₂ -2b₁ =2a₂ -2a₁ =2×thickness of wound layers (2×f) the minor axis 2b₁of the mandrel equals 122.5 mm and the minor axis 2b₂ of the ellipsedefined by the outer surface of the sleeve equals 372.5 mm.

One consequence of the cutting method described above is that the endsof the elliptical sleeve are not used. In the example of FIG. 5, aboutthirty parts can be obtained from a sleeve that is 1.5 meters (m) longresulting in a waste of about 350 mm at the sleeve ends. Clearly, thiswastage represents a smaller percentage if the sleeve is longer.

The present invention can also be carried out so as to further reducematerial losses, by using a sleeve with an elliptical cross sectionhaving planar faces at both ends which are inclined relative to a rightcross section of the sleeve, i.e. the faces are parallel to the cuttingplane envisaged for the sleeve. Thus parts can be cut from the entirelength of the sleeve and no unused material is left at the ends. Asleeve with such a shape can be produced by displacing the mandrel alongits axis with reciprocating motion during winding. Such a sleeve canalso be obtained by displacing the roller supplying the fibrous materialfor winding with reciprocating motion during winding, in a directionwhich is longitudinal relative to the mandrel. When a fairly smallsleeve cutting angle is used this method does not produce too great aperturbation as regards deformation of the sheet or the angle of thefibers relative to the friction surfaces of the parts cut from thesleeve.

The winding step is now described. When needling is carried outsimultaneously, this step must be applied to ensure that the needlingdensity is uniform. It means that the needle board and the mandrel areperiodically displaced relative to each other in order to compensate forthe eccentricity in the cross section of the mandrel.

FIG. 6 shows a first implementation of a winding method withsimultaneous needling of the wound layers. In this implementation, anelliptical mandrel 10 is driven in rotation about its axis S to wind afibrous material strip 1 around its periphery. At the same time, theaxis of mandrel S periodically moves towards and away from a needlingplane T along a rectilinear path Δ perpendicular to plane T. The curvedsurface of the outer layer wound on the mandrel is flush with needlingplane T, the contact region being a line M. A needle board 5 facing theouter surface of the wound layers is driven in reciprocating motionperpendicular to the needling plane T in order to strike and needle theouter layers wound on the mandrel.

The displacement of the mandrel as it describes a half-turn about itsaxis in this implementation is shown in FIG. 7. The ellipse Γ in thefigure represents the outer layers wound on mandrel 10. The limits ofthe displacement of axis S of the mandrel along line Δ are positions Aand B, axis S being at limit A when the major axis of ellipse Γ isperpendicular to needling plane T (positions 1 and 5 in FIG. 7) and axisS being at limit B when the minor axis of ellipse Γ is perpendicular toneedling plane T (position 3 in FIG. 7). As the axis of the mandrelmoves, the contact line M between the curved surface of the outer layerwound on the mandrel and needling plane T moves in this plane withreciprocating motion between two limits M_(e) (positions 2 and 4 in FIG.7).

Needle board 5 is also displaced with reciprocating motion in order tofollow the motion of contact line M in needling plane T.

In order to maintain needling at a constant density per unit area, andif the striking frequency of the needle board is constant, the relativetangential velocity between the needle board and the outer layer woundon the mandrel must be constant. The translational velocity (V_(T)) ofcontact line M in needling plane T can be kept constant by synchronizingthe rate of displacement of the mandrel axis between positions A and Band the rate of rotation of the mandrel about its axis of rotation.

For an ellipse Γ with a major axis 2a₂ and a minor axis 2b₂, thedistance between extremes M_(e) is 2(a₂ -b₂). If the ellipse is theright cross section of a cylinder with axis CC'(C being the center ofellipse Γ), it is preferable to move axis CC' using the translationalmotion described above. In this case, if the mandrel is rotated at arate ω(t), so that the velocity v_(T) is constant, a multiaxial sheet orfabric can be wound on the mandrel at a constant tangential velocity.

The invention provides a variation of this first implementation of amethod of winding with simultaneous needling of wound layers in whichthe rotational axis of the mandrel remains fixed. The relative motionrequired between the needle board and the mandrel is produced bydisplacing the needle board alone. In this case, needling no longertakes place in a single plane but in a series of mutually parallelplanes. In order to obtain a constant relative velocity at the moment ofcontact, the rate of needle board displacement is preferably keptconstant and the rate of rotation of the mandrel is varied.

Further, at each rotation of the mandrel, it must move away from theaverage position of the needle board so that the needling depth is keptconstant as the thickness of the preform increases during needling,since the needling stroke remains constant and equal to the thickness ofa few layers, for example. It is clear that this motion can be effectedby displacing the mandrel and/or the needle board. After winding thelast layer, a plurality of finishing needling passes can be carried outin order to keep the needling density constant in the layers locatedclose to the outer periphery, as described in FR-A-2 584 107 citedabove.

FIG. 8 shows a second implementation where an elliptical mandrel 10 isstill rotated about its axis S while the axis of the mandrelperiodically moves towards and away from needling plane T between twoextremes A, B along a rectilinear path Δ. This time, the motion of themandrel axis is such that the curved surface of the outer layers woundon the mandrel is tangential to needling plane T or the section along afixed line M. Needle board 5 comes into contact with the layers to beneedled in a region comprising this line M. Thus needle board 5 does notneed to be displaced in the needling plane. However, the needle boardhas an orientatable head which periodically oscillates in order to adaptits orientation to the direction normal to the curved surface of thelayers to be needled at line M. The orientation j of the needlingsurface of the needle board relative to plane T varies between twoextremes j_(e), where:

    φ.sub.e =arc tan[(a.sub.2 -b.sub.2)/√(a.sub.2 b.sub.2)]

2a₂ =major axis of the ellipse defined by the outer wound layer;

2b₂ =minor axis of the ellipse defined by the outer wound layer.

This second implementation can produce constant needling density perunit area by suitably controlling the rate of rotation of the mandrel,the rate of displacement of the mandrel axis in the direction Δ, and therate at which the needle board orientation is oscillated.

FIGS. 9 and 10 show a third implementation where the elliptical mandrelis still driven in rotation about its axis and, at the same time, themandrel axis is moved to follow a path Q formed by two half-ellipses(see FIG. 9). As a result of the motion of the mandrel, the outer curvedsurface of the wound layers remains tangential to the needling plane allalong fixed line M. This means that the needle board can strike over afixed region in the needling plane and it has a fixed orientationrelative to this plane. This solution is mechanically more complex tocarry out than the two previous solutions (see FIG. 10 which shows themovement of the ellipse defined by the outer wound layer as the mandrelmakes a half-turn about its axis). Nevertheless, it is easier to carryout if robotic means are used to control movement of the mandrel.

FIG. 11 shows apparatus for carrying out the first implementation of thewinding and needling method described above, in the variation where theaxis of the mandrel is not moved but the needle board is moved in orderto follow an elliptical path. In this case, contact between the needleboard and the outer wound layers takes place in multiple mutuallyparallel planes and the needle board is tangential to the surface to beneedled at the moment of contact.

The apparatus of FIG. 11 comprises a mandrel assembly 100, a needlingassembly 500 and a payout assembly 600, all positioned on a machinedplate 200 inserted in a floor 300.

The payout assembly 600 comprises a spool 6 of fibrous material strip 1wound around a central core. A geared motor 7 controls the rotation ofthe central core of spool 6 to pay out strip 1 and supply it to mandrelassembly 100. As paying out begins, strip 1 is entrained manually aroundthe mandrel until one complete turn has been made and the free end ofthe strip is trapped under the beginning of the second layer. A selvedgeguide cell 8 ensures that the strip is positioned properly relative tothe mandrel assembly during subsequent winding. A cell 9 for measuringthe diameter of spool 6 is provided beneath the spool in order tomeasure the change in diameter of the spool 6 during winding. Diametermeasuring cell 9 detects the diameter of spool 6 continuously orperiodically, for example using optical means. The signal produced bydiameter measuring cell 9 can be used to increase the distance betweenmandrel assembly 100 and needling assembly 500 as the thickness of thelayers wound on the mandrel increases.

In mandrel assembly 100, a low eccentricity elliptical mandrel 10 ismounted on a central shaft mounted on a frame 12. Frame 12 is mounted onplate 200 and is guided by a central guide 13 extending under plate 200so that it can be displaced to move it away from needling assembly 500as a function of the signal produced by diameter measuring cell 9 inpayout assembly 600. A geared motor 14 rotates the central shaft andthus the mandrel 10. In this implementation, geared motor 14 varies therate of rotation of mandrel 10 so as to keep the relative tangentialvelocity between the surface to be needled and the needle boardconstant.

The central shaft is arranged on the frame so that the central shaft,and thus the mandrel, can be moved in the longitudinal direction, i.e.along the axis of mandrel 10, the motion being periodic and used tooffset the strike position of the needle board slightly in order toprevent needling from always occurring in the same radial planes. Aroller 16 is applied against the outer wound layer on mandrel 10 to holdthe wound layers. In FIG. 11, an arrow A indicates the path followed byroller 16 during winding.

Needling assembly 500 comprises a needle board 5 extending across thewidth of fibrous material strip 1 to be needled and guided at a firstend of an extensible arm 20. The other end of the extensible arm ismounted on a housing 22. Arm 20 is forked at its first end and ispressed against the surface of the sleeve during needling by a cylinder23 bearing on housing 22. The needle board is caused to strike in thetransverse direction, i.e. the horizontal direction in FIG. 11. Thereciprocating motion of needle board striking is produced by aneccentric drive device 29 lodged in housing 22. In this embodiment, thestrike frequency of the needle board 5 is kept constant. Needlingassembly 500 also comprises an extractor 24 which can extract fiberparticles which may become detached from the fibrous strip duringneedling.

Housing 22 stands on a slide 25 displaceably mounted and guided on a rod13 so that it can move away from and towards mandrel assembly 100 underthe control of a geared motor 26. A further geared motor 28 controls thevertical displacement of housing 22. Geared motors 26 and 28 arecontrolled so as to produce a periodic elliptical motion of the needleboard in accordance with the variation of the first implementation ofthe winding and needling method described above. Geared motor 26 is alsocontrolled so as to keep the needling depth constant as the sleevebecomes thicker.

The methods and apparatus described above can be used to producepreforms for annular parts for a variety of uses, in particular brakedisks. The preform production methods described above and the choice ofconstitutive materials for the fibers can be adapted to the nature ofthe envisaged parts. For brake disk preforms, the choice of fiberorientation relative to the friction faces during winding can be afactor in optimizing the preforms.

As an example, if in the winding and needling methods described above,two-dimensional (0° and 90°) fabric (sheets or woven cloth) are wound sothat one direction is parallel to the feed direction of the sheet underthe needling head, and rings are cut with a cutting angle of 45°, brakedisks can be obtained in which the fibers are inclined relative to thefriction faces at an angle of 45°. When brake disks are produced frompreforms produced by such methods, shear stresses during service will nolonger be in an interstratum direction.

Further, winding multiaxial cloth or sheets of carbon yarns (2or 3directions) with two pre-determined yam directions, the angle of attackof the fibers relative to the friction faces of the envisaged brake diskcan be selected. The yams of one of the directions can be constituted bycontinuous carbon filaments, bonding by needling being ensured by rovingyams such as those in FR-A-2 669 940, positioned in the other direction(the needles are orientated suitably).

We claim:
 1. A method of producing preforms for producing annular partsof composite material, comprisingproviding a mandrel with an ellipticalcross section; and winding a strip of fibrous material on the mandrel insuperposed layers to produce a preform in the form of an ellipticalsleeve; obtaining annular parts by cutting the sleeve along planes whichare not perpendicular to an axis of the sleeve.
 2. A method according toclaim 1, further comprising needling the superposed layers to connectthe superposed layers to each other.
 3. A method according to claim 2,further comprising needling each new layer of the fibrous material beingwound around the mandrel simultaneously with its winding around themandrel, using a needle board extending over the width of the strip. 4.A method according to claim 3, wherein during the winding and needlingstep a needling surface of the needle board is oriented tangentially toa next portion of the fibrous material to be needled at a moment ofcontact between the mandrel and the needle board.
 5. A method accordingto claim 4, wherein at least a portion of a most outwardly disposed,previously wound layer of the fibrous material is always tangential to aneedling plane (T) along the length of a contact line (M) during thewinding and needling step.
 6. A method according to claim 5, furthercomprising displacing the needling plane by reciprocating motions of themandrel and the needle board during the winding and needling step.
 7. Amethod according to claim 5, wherein the position of the contact line inthe needling plane remains fixed, and further comprising moving themandrel such that an axis of the mandrel is periodically moved along apath comprising two half-ellipses during the winding and needling step.8. A method according to claim 4, further comprising moving the mandrelsuch that a portion of the most outwardly disposed, previously woundlayer is always in a fixed needling position and the needling surface ofthe needle board moves to remain tangential to the next portion of thefibrous material to be needled, during the winding and needling step. 9.A method according to claim 6 further comprising rotating the mandrelabout its axis and, at the same time, displacing the axis of the mandrelwith respect to the needling plane, using the reciprocating motion ofthe mandrel during the winding and needling step.
 10. A method accordingto claim 2, wherein the needle board strikes at a constant frequencyduring the winding and needling step.
 11. A method according to claim 2,further comprising increasing an average distance between the mandreland the needle board as a thickness of wound layers of the fibrousmaterial increases.
 12. A method according to claim 2, wherein the endsof the mandrel are planar faces which are mutually parallel and inclinedrelative to a right cross section of the sleeve, and wherein during thewinding and needling step there is a relative displacement between thesleeve and the strip of the fibrous material being wound, said relativedisplacement provided by a reciprocating motion of the mandrel in alongitudinal direction of the mandrel.
 13. A method according to claim1, further comprising cutting the sleeve along a cutting angle, whereinthe cutting angle is selected so that each of the rings resulting fromthe cutting have a circular cross section at an inner periphery or at anouter periphery, or at a predetermined position between the innerperiphery and the outer periphery.
 14. A method according to claim 13,wherein the cutting angle is selected so that rings resulting from thecutting are circular at their half-width.
 15. A method according toclaim 1, wherein the cutting step is performed after at least one sleevedensification step.
 16. A method according to claim 15, wherein duringthe at least one sleeve densification step the sleeve is densified bytemperature gradient chemical vapor infiltration.
 17. A method accordingto claim 1, further comprising machining a periphery of the annular partto produce a brake disk.
 18. A method according to claim 17, wherein thestrip of fibrous material is aligned with a feed direction of themandrel.
 19. A method according to claim 8, further comprising:rotating,during the winding and needling step, the mandrel about its axis and, atthe same time, displacing an axis of the mandrel with a reciprocatingmotion to approach and move away from the plane or needling position;striking, during the winding and needling step, the needle board againstthe strip of fibrous material at a constant frequency; and increasing,during the winding and needling step, the average distance between themandrel and the needle board as the thickness of the assembly of woundlayers increases.
 20. A method according to claim 19, whereinthe ends ofthe mandrel are planar faces which are mutually parallel and inclinedrelative to a right cross section of the sleeve, and further comprising,during the winding and needling step, providing a relative displacementbetween the sleeve and the strip of fibrous material, said relativedisplacement provided by a reciprocating motion of the mandrel in thelongitudinal direction of the mandrel.
 21. A method according to claim19, further comprising:selecting a cutting angle of the sleeve such thatthe cut rings resulting from the cutting have a circular cross sectionat their inner periphery or at their outer periphery, or at apredetermined position between the inner and outer peripheries, or attheir half-width; and providing, prior to cutting the sleeve, at leastone sleeve densification step, wherein the sleeve is densified bytemperature gradient chemical vapor infiltration during the at least onesleeve densification step.
 22. A method according to claim 20, furthercomprising:selecting a cutting angle for cutting the sleeve such thatthe rings resulting from the cutting have a circular cross section attheir inner periphery or at their outer periphery, or at a predeterminedposition between the inner and outer peripheries, or at theirhalf-width; and providing, prior to cutting the sleeve, at least onesleeve densification step, wherein the sleeve is densified bytemperature gradient chemical vapor infiltration during the at least onesleeve densification step.
 23. A method according to claim 21, furthercomprising: machining an inner and/or an outer periphery of the annularpart; andwherein the strip of fibrous material is aligned with a feeddirection of the mandrel during the winding step.
 24. A method accordingto claim 22, further comprising:machining an inner and/or an outerperiphery of the annular part; and aligning, during the winding step,the strip of fibrous material with the feed direction of the mandrel.25. Apparatus for producing preforms for producing annular parts ofcomposite material, comprising:a mandrel having an elliptical crosssection; and means for winding a fibrous material strip on the mandrelin superposed layers to produce a preform in the form of an ellipticalsleeve.
 26. Apparatus according to claim 25, further comprising a needleboard for striking layers of the fibrous material strip while they aresimultaneously being wound onto the elliptical mandrel, therebyconnecting the layers together by needling.
 27. Apparatus according toclaim 26, further comprising means for moving the needle board and/orthe mandrel such that a needling surface of the needle board is in aplane tangential to a next portion of the fibrous material strip to bewound and a most outwardly disposed, previously wound layer of thefibrous material strip, at the moment of mutual contact between the nextportion of the fibrous material strip to be wound and the most outwardlydisposed, previously wound layer of the fibrous material strip. 28.Apparatus according to claim 27, characterized in that the means formoving the needle board and/or mandrel maintain a portion of the mostoutwardly disposed, previously wound layer of the fibrous material stripin a tangential orientation to a needling plane along the length of acontact line.
 29. Apparatus according to claim 28, wherein the means formoving the needle board and/or the mandrel further comprises means formoving the needle board and/or the mandrel such that the contact line inthe needling plane is displaced along the needling plane withreciprocating motion of the needle board.
 30. Apparatus according toclaim 28, wherein the means for moving the needle board and/or themandrel moves the needle board and/or the mandrel such that an axis ofthe mandrel is periodically moved along a path comprising twohalf-ellipses so that the position of a contact line in the needlingplane remains fixed.
 31. Apparatus according to claim 27, wherein themeans for moving the needle board and/or the mandrel moves the mandrelsuch that a portion of the most outwardly disposed, previously woundlayer of the fibrous material strip is maintained in a fixed needlingposition and the needling surface of the needle board is maintainedtangential to the next portion of the fibrous material strip to be woundand the most outwardly disposed, previously wound layer of the fibrousmaterial strip.
 32. Apparatus according to claim 28, wherein the meansfor moving the needle board and/or the mandrel include means for movingan axis of the mandrel with reciprocating motion to approach and moveaway from needling plane or position.
 33. Apparatus according to claim25, further comprising means for striking the fibrous material stripwith the needle board at a constant frequency.
 34. Apparatus accordingto claim 25, further comprising means for increasing an average distancebetween the mandrel and the needle board as a thickness of wound layersof the fibrous material strip increases.
 35. Apparatus according toclaim 25, wherein the ends of the mandrel are planar faces which areparallel to each other and not perpendicular to an axis of the sleeve,and further comprising means for controlling a relative displacementbetween the mandrel and the strip of fibrous material during winding,the relative displacement provided by a reciprocating motion in alongitudinal direction of the mandrel.
 36. Apparatus according to claim32, further comprising:means for striking the needle board against thestrip of fibrous material at a constant frequency; means for increasingan average distance between the mandrel and the needle board as athickness of wound layers of the strip of fibrous material increases;wherein the ends of the mandrel are planar faces which are parallel toeach other and not perpendicular to an axis of the sleeve; and means forcontrolling a relative displacement between the mandrel and the strip offibrous material during winding, the relative displacement provided by areciprocating motion of the mandrel along a longitudinal direction ofthe mandrel.